Please convince me to get Rdio

I've been trying to convince myself to get the $9.99 monthly rdio membership but I just can't. Few things stopping me from getting it.

1. I am on T-Mo with a 5GB plan and than throttled after that, so first of all I'm not sure how much data rdio will use.

2. Offline music. I don't understand exactly how this works. I'm guessing "sync to mobile" makes that song for offline listening.

I currently have 6 days left of my trial so we'll see how this goes. The main thing that I'm worried about it the amount of data that it will use. Any insight on that?